Recently, there has been investigated the use of carbonaceous fibers as an electron source of an electron-emitter in a cold cathode employed in a device such as an electronic display device or an imaging device.
With regard to processes for producing such carbonaceous fibers, Japanese Patent Application Laid-Open (Kokai) No. 8-115652, for example, discloses carbonaceous fibers deposited through pyrolysis of hydrocarbon gas serving as a raw material in a microcavity forme d between two electrodes, each electrode being disposed on an insulated substrate.
Japanese Patent Application Laid-open (kokai) No. 10-112257 discloses a process for gas phase synthesis of diamond-like carbon, including steps of implanting carbon ions or carbon cluster ions on a substrate cathode surface to thereby form nucleating sites, and growing diamond-like carbon from the sites.
Although these processes can be carried out from a technical standpoint, the processes involving a thermal treatment step, however, have an adverse effect on the cathode material. Therefore, heat treatment of the formed carbonaceous fibers is not an acceptable technique for limiting the emission of electrons.
Moreover, since these processes involve direct formation of carbon on a substrate such as a cathode, mass production requires unique know-how, as well as special facilities and manufacturing techniques. Thus, due to such requirements, manufacturers of cathode materials generally do not employ such processes.
In recent years, carbon nanotubes having a diameter of some 10 nm or less have been studied as an electron-emitting material. A carbon fiber nanotube is a tube formed of graphite and typically has a diameter of 1-50 nm. Such a product can be formed by deposition on an electrode by arc discharge of a carbon electrode or by application of a high-intensity laser beam to a carbon electrode in a suitable atmosphere. The nanotube typically has one sharp end. See, for example, Chemistry Today, p.57, July, 1998.
Carbon nanotubes have chemical stability and high mechanical toughness, and applications thereof as electron sources for field emission are currently being investigated. For example, Saito et al. disclose in Ceramics, 33, (1998), No. 6, a fluorescent display device in which a number of carbon nanotubes are attached to a cathode plate. The authors indicate possible use of carbon nanotubes in a display device such as a low-power planar display device or an ultrafine color CRT.
However, no suitable industrial process for producing carbon nanotubes has yet been established, and thus inexpensive carbon nanotubes of stable quality have not been available in commercial quantities.
Recently, vapor-grown carbon fibers having a structure similar to that of carbon nanotubes and with a diameter on the order of several microns have been produced on a large scale. As disclosed in Japanese Patent Publication (Kokoku) No. 04-24320 and Japanese Patent No. 2778434, the above type of carbon fiber is produced by spraying an organic compound in a reactor to thereby pyrolyze the compound. Precise examination of the thus-obtained fiber has revealed that the fiber is composed of planes of carbon atoms having a condensed ring structure concentrically grown around the longitudinal axis of the fiber. The fiber has a spherical closed end or a cut end having a cross-section in a plane approximately normal to the fiber axis.
If the fiber has a sharp end, carbon atoms of a condensed ring structure appear in the edge plane, to thereby enhance the field emission characteristic of the carbonaceous fiber when used as a field emission source.
However, a carbonaceous fiber having two sharp ends has not been found. Use of such a carbonaceous fiber as an electron-emitting material is expected to have an effect of increasing emission efficiency.